1. Field of the Invention
The present invention relates to off-chip drivers, and more particularly, to off-chip drivers including a delay stage, and methods for controlling the delay stage to control the slew rate.
2. Description of the Prior Art
An off-chip driver is a well-known circuit in the field of electrical technology. One key characteristic of an OCD is the slew rate of the output. Conventional OCD employ different methods to control the slew rate. Among these methods is one that includes delay stages for more accurately controlling slew rate.
In an example circuit, the pre-driver is divided into two branches, as is the final driver. The time domain stage (delay stage) includes two sets of delay elements, comprising transistor elements for providing a voltage signal in response to a data in (logic) signal, and buffers, for buffering the voltage signals, wherein the output of the delay stage is input to the pre-drivers. A first voltage signal is buffered in a first buffer, and is utilized to turn on the first branch of the pre-driver, which controls the first branch of the final driver; a second voltage signal is buffered in a second buffer, and is utilized to turn on the second branch of the pre-driver, which controls the second branch of the final driver.
The delay elements help to provide a more gradual controlled response at the output. Although the utilization of delay elements helps to more accurately control the slew rate of an OCD, the buffers are susceptible to process, voltage and temperature (PVT) variations. If there is some variation in the external conditions, the delay between these branches changes, and therefore the slew rate will be affected. In the current scheme these variations push each stage in the same direction, whereby the effect is amplified. In the example circuit mentioned, if the supply voltage was lowered the output stage would operate slower. Additionally, with a low supply voltage the delay between the branches would also be larger. Both of these effects go on to reduce the slew rate.